A battery cell is a useful article that provides stored electrical energy that can be used to energize a multitude of devices, particularly portable devices that require an electrical power source. The cell is an electrochemical apparatus typically formed of at least one ion-conductive electrolyte medium disposed between a pair of spaced-apart electrodes commonly known as an anode and a cathode. Electrons flow through an external circuit connected between the anode and cathode. The electron flow is caused by the chemical-reaction-based electric potential difference between the active anode material and active cathode material. The flow of electrons through the external circuit is accompanied by ions being conducted through the electrolyte between the electrodes.
Electrode and electrolyte cell components typically are chosen to provide the most effective and efficient battery for a particular purpose. Lithium is a desirable active anode material because of its light weight and characteristic of providing a favorable reduction potential with several active cathode materials. Liquid and aqueous electrolytes have often been chosen because of favorable ion-conducting capabilities. Despite the benefits provided by certain anode materials and electrolytes, the materials themselves and, often, the combination of a particular electrode material and a particular electrolyte can cause problems in cell performance and, in some instances, can create a hazardous condition. For example, as advantageous as lithium can be as an active anode material, it can be degraded and otherwise react undesirably with such common mediums as air and water, and certain solvents. As a further example of problems, certain liquids that are useful as effective electrolytes can create hazardous conditions when serving as components of a lithium-ion battery.
For the reasons broadly stated above, it is often desirable to use a non-aqueous and non-liquid electrolyte medium in cells. Non-aqueous electrolyte mediums are desired because water can interact undesirably with some desirable electrode materials such as lithium. Non-liquid electrolyte mediums are desired for several reasons. One reason is that liquid electrolytes often react detrimentally with desirable electrode substances such as lithium even though the liquid is non-aqueous. Another reason that liquid electrolytes can be undesirable is the need to prevent electrolytic material from freely flowing beyond a predetermined geometric boundary configuration. For example, leakage of electrolyte solution from the battery container is typically undesirable. Another problem with liquid electrolytes is that some solvents that are used as effective non-aqueous, liquid electrolytes are flammable and have a relatively high vapor pressure. The combination of flammability and high-vapor pressure creates a likelihood of combustion. Further in this regard, batteries that use lithium-based anodes can pose severe safety issues due to the combination of a highly volatile, combustible electrolyte and the active nature of lithium metal.
Some of the problems associated with particular cell electrodes and electrolyte can result in internal failure of the cell. One type of internal failure is the discharge of electric current internally, within the cell, rather than externally of the cell. Internal discharge may also be referred to as “self-discharge.” Self-discharge can result in high current generation, overheating and ultimately, a fire. A primary cause of self-discharge has been dendritic lithium growth during recharge of a rechargeable battery. In rechargeable cells having lithium anodes, dendrites are protuberances extending from the anode base that are formed during imperfect re-plating of the anode during recharge. Dendrites or growths resulting from low-density lithium plating during recharge can grow through the separator that separates anode from cathode particularly if the separator is porous or solid but easily punctured by the growth. When the growths extend far enough to interconnect the anode and cathode, an internal electrical short circuit is created through which current can flow. Electrical current produces heat that will vaporize a volatile electrolyte substance. In turn, vaporization of the electrolyte can produce extreme pressure within the battery housing or casing which can ultimately lead to rupture of the housing or casing. The temperatures that result from an electrical short circuit within a battery are sometimes high enough to ignite escaping electrolyte vapors thereby causing continuing degradation and the release of violent levels of energy. Lithium-ion batteries were developed to eliminate dendritic lithium growth by utilizing the lithium ions inserted into graphite anodes rather than re-platable lithium metal anodes. Although these lithium-ion batteries are much safer than earlier designs, violent failures still occur.
Ion-conductive, solid-glass electrolytes and ceramic electrolytes have been developed in the past to address the need for an electrolyte medium without the shortcomings described above. These solutions have included glass electrolyte materials such as Lithium Phosphorous Oxy-Nitride (LiPON) and a class of glass-ceramic materials generally referred to as LiSICON (an acronym for Lithium Super-Ionic Conductor) structure-type materials and NaSICON (an acronym for Sodium Super-Ionic Conductor, wherein the “Na” portion of the acronym is the chemical symbol for sodium) structure-type materials. However, these materials have limitations. LiPON has low ionic conductivity, in the range of 1.2E-6 S/cm, and generally can only be applied or used as thin films less than 10 μm thick. In addition, it has to be produced using a reactive sputtering process in a low vacuum environment which can be very expensive. LiPON is also unstable in contact with water which eliminates its possible use as a protective electrolyte in battery systems where exposure to moisture or ambient air may occur. On the other hand LiSICON and NaSICON structure-type materials are stable in contact with water but are unstable in contact with lithium. When in contact with lithium this class of materials turns dark and can conduct electric current by electron flow thus minimizing usefulness as electrolyte separators.
Thus it can be appreciated that it would be useful to have a cell electrolyte medium that is a conductor of ions, that is protective of and stable in contact with lithium, that is non-aqueous, that is non-liquid, that is non-flammable and that does not produce short circuits that are associated with dendritic plating of lithium.